1. Field of the Invention
The present invention relates to an image pickup apparatus and automatic level adjusting method applicable to a digital still camera and the like, and more particularly to an image pickup apparatus and automatic level adjusting method capable of suppressing flicker generated in fluorescent lighting.
2. Description of Related Art
When using a CMOS sensor as a solid-state imaging device of an image pickup apparatus such as a digital still camera for taking still pictures, bright and dark horizontal striped patterns can occur in pictures taken in an environment in which brightness varies periodically such as in the fluorescent lighting. They arise from the difference between the timing of the flickering of the fluorescent lighting and that of the photoelectric conversion of individual pixels of the pictures. This phenomenon is referred to as flicker.
To suppress the flicker, there are such conventional methods as averaging pickup data over several past frames; or setting charge storage time such that it becomes an integer multiple of the flickering period of a light source. An example of the latter is disclosed in Japanese patent application laid-open No. 2000-32330. This method carries out the level adjustment as follows: When the subject illuminance reduces and the charge storage time of the imaging device becomes longer than 1/(flickering frequency of the light source) second, this method limits the allowed charge storage time to an integer multiple of 1/(flickering frequency of the light source) second. As for the fraction of the charge storage time less than the adjustable step by the integer multiple of the 1/(flickering frequency of the light source) second, the level adjustment is carries out by controlling the gain of an amplifier of the image pickup apparatus.
FIG. 14 is a block diagram showing a configuration of an automatic level adjusting apparatus for implementing the conventional automatic level adjusting method disclosed in the foregoing Japanese patent application laid-open No. 2000-32330. In FIG. 14, the reference numeral 101 designates an imaging device; 102 designates a correlated double sampling circuit (CDS); 103 designates an analog amplifier; 104 designates an A/D converter; 105 designates a digital amplifier; 106 designates a frame average calculating circuit; 107 designates an electronic shutter speed controller; 108 designates a digital amplifier controller; and 109 designates a timing signal generator for generating a timing signal for operating the electronic shutter of the imaging device.
Next, the operation of the conventional automatic level adjusting apparatus will be described.
An analog video signal captured by the imaging device 101 passes through the noise elimination by the CDS 102, and is amplified by a fixed factor by the analog amplifier 103. The signal output from the analog amplifier 103 is converted into a digital signal by the A/D converter 104. The digital signal is amplified by the digital amplifier 105, and transferred to a camera signal processor at the next stage not shown.
On the other hand, the output of the digital amplifier 105 is supplied to the frame average calculating circuit 106, and its calculation result is supplied to the electronic shutter speed controller 107 and digital amplifier controller 108. The electronic shutter speed controller 107 and digital amplifier controller 108 determine the electronic shutter speed and the gain of the digital amplifier for controlling the charge storage time of the imaging device 101 through the timing signal generator 109, and the amplification factor of the digital amplifier 105.
Next, the conventional automatic level adjusting method will be described in more detail. FIG. 15 is a diagram illustrating the conventional automatic level adjusting method concretely.
As illustrated in FIG. 15, the flickering frequency of the light source is assumed to be 1/100 second. Initially, assume that the output is set at a specified level in the condition that the subject illuminance is high, the electronic shutter speed is fastest, and the gain of the digital amplifier 105 is minimum (+0 dB). If the subject illuminance reduces from the condition, the output level falls below the specified level. In this case, the electronic shutter speed is made slower to increase the output level. If the output is still less than the specified level even when the electronic shutter speed becomes 1/100 second, the gain of the digital amplifier 105 is increased with maintaining the electronic shutter speed at 1/100 second. If the output is still less than the specified level even when the gain of the digital amplifier 105 becomes maximum (a little less than +6 dB), the electronic shutter speed is set at 1/50 second, and the gain of the digital amplifier 105 is adjusted at the minimum (+0 dB). If the output is still less than the specified level, the gain of the digital amplifier 105 is increased up to the maximum (a little less than +6 dB) with maintaining the electronic shutter speed at 1/50 second.
With the foregoing configuration, the conventional automatic level adjusting method has a following problem depending on the frequency of a power supply: Although the illuminance fluctuation period of a fluorescent lamp is 1/100 second in a region where the frequency of the power supply is 50 Hz, that in the region where the frequency of the power supply is 60 Hz, it is 1/120 second. Thus, the illuminance fluctuation period of the fluorescent lamp varies depending on the frequency of an alternating power supply, and this presents the main cause of the flicker. As a result, although the flicker is suppressed in one region, it cannot be suppressed in another region because of the different illuminance fluctuation period.